PSI Power & Auxiliary Transformer — Maintenance & Periodic Overhaul

Power Transformer — Monthly Schedule

• Maximum temperature of transformer oil AND winding on dial indicator: check, compare with previous values, investigate abnormal change, and reset indicator.
• Oil level in conservator (MOLG): check per the Oil Temperature Indicator (OTI); top up with filtered oil if low.
• Buchholz Relay: check for gas collection — if gas collected, send for DGA per RDSO guideline (Annex 2.04 of ACTM Vol II, Part I).
• Oil level in OIP condenser bushing: check via oil level indicator. If sealed bushing shows less-than-minimum oil, measure Tan Delta and capacitance and compare with earlier values — MAX ALLOWABLE Tan Delta is 0.007, capacitance limit is 110% of the factory-set value.
• Tap changer: check/record position of standby and service transformer.
• Tank, radiators, conservator, bushing, oil level indicator, gauges: check/clean dirt, leakage, cracks — attend if crack/leakage observed.
• Dehydrating breather: check for choking (insect build-up/dirt); check gasket intactness and silica gel colour — if silica gel is pink, replace with new gasket and dry silica gel (or recondition). If silica gel is too wet, check BDV of transformer oil — IF BDV IS LESS THAN 50kV, FILTER THE OIL TILL IT REACHES 50kV. Check/top up oil in the breather oil cup.
• Heater in marshalling box: check for proper functioning — check and rectify connection if not working.
• Sound: check for abnormal humming, find the reason if observed.
• All external connections: check visually for normal condition without discoloration from local heating — clean and tighten bolts/nuts if any heating sign.

Power Transformer — Half-Yearly Additions

• Test oil sample with oil test kit for BDV and acidity — compare and act per Annexure I.
• PRD/explosion vent: check for damage and presence of oil; check connection and operation — replace damaged PRD with new one.

Insulation Resistance & Polarization Index (PI) — The Key Half-Yearly Test

Insulation resistance is measured for 10 seconds, 60 seconds, and 600 seconds for HV-LV, HV-E, and LV-E, using a 2.5kV or 5kV megger — with the transformer disconnected and bushings clean/free from moisture, and temperature at which IR is recorded also noted. PI (Polarization Index) is calculated as the ratio R60/R10, and also R600/R60. IR values are compared with the last recorded value.

Power Transformer — Yearly Additions

• Infrared temperature scanning of tank, bushings, tap changer, radiator, and connection joints — preferably at full load, compared with previous scan results.
• OIP condenser bushing: Tan Delta, capacitance, and IR measured/compared (Schering bridge may be used) — bushing must be cleaned of dust/moisture, and test done on a SUNNY DAY. Max allowable Tan Delta 0.007, max capacitance 110%.
• Test oil per IS-1866-2000 (Annexure I / ACTM Vol II Part-I, Annexure 2.03B) — replace oil if it doesn't meet requirements.
• Dissolved Gas Analysis (DGA) per RDSO guideline (Annexure 2.04, ACTM Vol II Part I) — compare with guideline; IF DGA VALUES SHOW PAST ABNORMALITY, DGA TEST FREQUENCY INCREASES TO EVERY SIX MONTHS.
• Working of tap changer switch: move up/down full range for self-wiping action to ensure good contacts; set the tap finally at the correct position.
• Buchholz relay: check operation, measure IR by 500V megger (should be NOT LESS THAN 20 MEGA OHM), and continuity test for contacts with test lock screw set at 5° — relay must operate on continuity test.
• Voltage ratio test with Wheatstone bridge/voltmeter: compare trends with pre-commissioning/factory/earlier values — a SUDDEN variation from the last measured value calls for further investigation.
• Winding resistance test with Wheatstone bridge voltmeter-ammeter (DC current, on principal tap; measured value converted to 75°C) — this test should be done LAST among winding tests, to avoid residual DC flux affecting other test results. Deviation from pre-commissioning/factory value should be LESS THAN ±5%.

Auxiliary Transformer — Monthly, Quarterly, Half-Yearly, Yearly

• Monthly: visual inspection for oil leakage/overheating; clean externally; check silica gel colour (replace/reactivate if pink) and top up breather oil cup; check/clean/tighten HT and LT bushing terminal connections; top up oil in conservator with new filtered oil (BDV must be more than 30kV); clean AT enclosure of vegetation; check pole-mounted ICDP switch/control panel/fuses (fuse wire: 20 SWG for 34A, 22 SWG for 24A); check caution board and anti-climbing device availability; check/tighten earthing connections; check arcing horn on HV bushing for flashover marks.
• Quarterly (additional): check condition of Drop-Out (DO) fuse barrel/element, replace breakage/overheating/non-standard fuse wire with STANDARD fuse wire; check for any unusual sound (equipment should be soundless).
• Half-Yearly (additional): BDV of oil (sample from bottom of tank, 2.5mm spindle gap) MORE THAN 30kV; measure additional arcing horn gap on fixed 9-ton insulator — MINIMUM 165mm, record; measure insulation resistance HV-Earth ≥200MΩ (2.5kV megger), HV-LV ≥200MΩ (2.5kV megger), LV-Earth ≥2MΩ (500V megger); measure earth resistance — NOT MORE THAN 10 OHMS; measure IR value of LT cable (AT to ICDP switch, ICDP switch to control panel) ≥2MΩ at 30°C (500V megger).
• Yearly (additional): winding continuity on all tappings (multimeter/500V megger) — should be ZERO; check smooth operation of tap changer on all tappings (no obstruction); check oil acidity — MAXIMUM 0.5 mg KOH/gm; ratio test (apply AC to HV winding, measure LV voltage, compare with previous reading); check HT/LT jumpers and lugs for looseness/flash marks — replace if found; painting (if faded/rusted) with epoxy and polyurethane paints per RDSO spec ETI/PSI/118 (A&C slip no.8); replace DO fuse wire with new one.

Periodic Overhaul (POH) — Power Transformer

The procedure applies to bell-type tank construction transformers. POH can be done at site (TSS), in the traction repair shop, or through trade. The transformer must NOT be opened during rainy/cloudy weather for inspection/repair/overhaul.

Tests BEFORE Undertaking POH

1. Measure dielectric strength (BDV), acidity, water content (ppm), and Tan Delta of transformer oil.
2. Dissolved Gas Analysis (DGA) of transformer oil.
3. Measurement of winding resistance on all taps.
4. Measure insulation resistance of winding and Polarization Index.
5. Measurement of Tan Delta and capacitance of condenser bushing.

All these pre-POH values are recorded and compared with values recorded AFTER completion of overhauling.

Steps for Overhauling

Untanking: drain out oil, disconnect all leads, remove manhole covers/accessories/conservator/breather/radiator/fittings/HV/LV bushings and store safely. One set of standard-quality gaskets must be available for replacement (per Annexure IX). The bell tank is removed by suitable crane/winch, carefully to avoid damaging active parts. Defects observed during inspection are recorded, and issues like tank/conservator/radiator leakage are permanently rectified during POH.

Drying-Out/Filtration: vacuum drying is done per IS 1866:2000, MAINTAINING PRESSURE LESS THAN 0.10 TORR and MAXIMUM TEMPERATURE OF 60°C. During filtration/drying, temperature and IR value readings are recorded EVERY TWO HOURS from commencement till full operation is completed (per ACTM Vol II Part I, para 20208/20210). To confirm moisture removal, PI after drying should preferably be EQUAL TO OR MORE THAN 2.

• Items checked/attended during overhauling: insulating oil, windings and internal parts, conservator/radiators/metal parts and pipes, drain/oil-sampling/filter valves, insulation blocks/permali wood packing/spacers/wedge/keyed blocks (checked for displacement from repeated short-circuit stress), pressure screws with belleville washers, top/bottom yoke bolts (ferrule insulation dielectric strength tested by 2kV AC for 1 minute), electrical joints/earth connections/tap changer lead connections, active-part-to-tank clearances, off-circuit tap changer contacts and operation, all fastener/core bolt tightness, condenser bushing oil level/cracks/leakage, and control cables.
• PRECAUTION: tools/objects (spanners, washers, cloth pieces) left inside a transformer present a short-circuit hazard — tools must be listed at the START of work and accounted for at the END.

Checks/Tests AFTER Overhauling

1. Voltage ratio test (voltmeter/ammeter/lamp or ratio meter).
2. Winding resistance test (Wheatstone bridge or voltmeter-ammeter method).
3. Magnetizing current test (voltmeter, ammeter, lamp).
4. IR of windings and Polarization Index with a motorised 5kV megger.
5. Transformer oil tests: BDV, ppm, Tan Delta, acidity.
6. Check protection equipment (Buchholz, PRD, MOLG, WTI, OTI) for proper working.
7. Check all gasket joints for leakage; check relay/alarm contacts for operation.
8. Check control/terminal boxes for water-tightness (replace gasket if required); check dial-type OTI/WTI for pointer freedom.
9. Check IR value and secondary winding resistance of bushing CTs.
10. Bushing Tan Delta/capacitance test — compare with prior values; MAX ALLOWABLE Tan Delta 0.007, capacitance 110% of factory-set value.
11. Check Tap Changer (TC) operates correctly in Manual, Local (electrical), and Remote electrical modes.
12. Check oil level in tank/conservator/condenser bushing/TC; check OTI/WTI pockets and breather oil cup, replenish oil; check arcing horn gap per drawing.
13. Check all earthing connections per diagram plate; check air delivery cooler fans work in right direction (where provided).
14. Ensure silica gel is active and blue, oil is at correct level in cup; check thermometer oil pockets are filled properly.
15. Remove/clear all tools and equipment used in POH from the transformer; lock the roller to prevent rail movement; release trapped air through air release plugs/valves.

Commissioning after POH follows ACTM Vol II Part I, para 20905/20906/20907. Performance is evaluated by comparing post-POH insulation resistance/PI and oil BDV/water content/Tan Delta/acidity/DGA values against pre-POH readings — all kept on record for ongoing performance monitoring.

Items Requiring Replacement WITHOUT FAIL During POH

1. Insulating oil.
2. Gasket and rubber items (per Annexure IX).
3. Pre-compressed board used to insulate the bottom of the tank from active parts.
4. Silica gel.

Other items (insulation material like permali wood packing, coil clamping screws/plates, radial paper covered insulation, fibre glass ferrule, and winding items like locking/plain spacers, keyed blocks, core packing, clamping rings, plus cable clamps/glands/lugs, relays/switches, and tap-changer parts like the Bakelite phase board, brass slip rings, copper brush assembly, gears/bearings, and connection cables) are replaced ON A CONDITION BASIS — only if found defective/worn, not automatically like the four items above.

Periodic Overhaul — Auxiliary Transformer

AT POH can be done in the repair shop or through trade. General cleaning first (remove dirt around tank/radiator/HV & LV bushing/neutral link block covers; blow dust off with compressed air), then a full visual inspection (tank deformation/cracks, bushing cracks/chipped discs, silica gel condition, oil leakage points, valve thread condition, gaskets, conservator/oil-level glass, arcing horn condition) with all defects noted.

• Tests BEFORE opening the transformer: insulation resistance (HV-Earth by 2.5kV megger, LV-Earth by 500V megger, HV-LV by 2.5kV megger); continuity test of primary/secondary windings (multimeter); tap changer condition/operation at all tap positions; BDV of oil (automatic BDV kit); Polarization ratio (2500V DC applied continuously, IR noted at 10 sec and 60 sec); DGA of oil; further oil tests (specific resistance/resistivity, water content, Tan Delta, flash point, neutralization value, sludge); ratio test.
• Before the physical work: drain oil, disconnect all leads, remove tank top cover/accessories/conservator/fittings, disconnect HV & LV bushings (store safely), disconnect earth leads from the primary earth bushing terminal link block, disconnect power cable connections from the secondary winding bushing.
• AFTER opening: measure insulation resistance of winding; check all core support bolts for cracks (replace if required); check condition of core/winding/lamination; check for water content/sludge accumulation inside the tank; check earth connections; check condition of soldered joints; check insulation paper for damage (re-insulate if necessary); check condition of tap changer contact tips at all tap positions.